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Abstract

A detailed characterization of the coherent x-ray wavefront produced by a partially illuminated Fresnel zone plate is presented. We show, by numerical and experimental approaches, how the beam size and the focal depth are strongly influenced by the illumination conditions, while the phase of the focal spot remains constant. These results confirm that the partial illumination can be used for coherent diffraction experiments. Finally, we demonstrate the possibility of reconstructing the complex-valued illumination function by simple measurement of the far field intensity in the specific case of partial illumination.

Figures (6)

(a) Sketch of a partially illuminated Fresnel Zone Plate. A pair of slits defines a rectangular aperture matching the transverse coherence lengths of the x-ray beam. (b) 2D schematic of the propagation of the wavefield produced by a partially illuminated FZP. The slits, placed 1.15 m upstream the lens, used to define the illuminated area, the FZP, the CS and the OSA are represented. The effective direction of propagation is tilted with respect to the FZP axis due to the lateral displacement of the slits.

Simulated complex field at the focal plane of a Fresnel Zone Plate for different conditions of partial illumination. Color rendition of the complex-valued probe for (a) 120v × 40h, (b) 60v × 20h and (c) 40v × 20hμm2 slits aperture. The phase is represented by colors and the amplitude (in logarithmic scale) by colors intensities.

Section of the simulated complex field in the direction of propagation for different conditions of partial illumination. Amplitude (expressed in logarithmic scale) for an opening of (a) 120v × 40h, (b) 60v × 20h and (c) 40v × 20hμm2.

(a) Measured intensities in logarithmic scale of the illumination probe at the detector. It is compared to (b) simulated intensities with the slits distant 1.15 m from the lens plane and (c) close (neglecting the propagation from the slits) to the FZP. The best agreement is obtained with a slit opening of 72v × 28hμm2. The non-zero intensity pixels in the center of the detector are due to direct beam photons transmitted by the central stop.

(a) Color rendition of the reconstructed complex-valued field at the focal plane compared to (b) the calculated propagation of Fig. 4b back to the focal plane. The phase is represented by colors and the amplitude (in logarithmic scale) by colors intensities.